Method and control system for operating a solar power tower system

ABSTRACT

A solar energy collection system includes a primary solar receiver and a secondary solar receiver. The secondary solar receiver generates steam using energy from solar radiation incident thereon. The primary solar receiver receives the generated steam from the secondary solar receiver and superheats the steam using energy from solar radiation incident thereon. A plurality of heliostat-mounted mirrors reflects incident solar radiation onto one of the primary and secondary solar receivers. A controller aims a portion of the heliostat-mounted mirrors at the primary solar receiver such that a predetermined thermal profile is provided on a surface of the primary solar receiver.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional PatentApplication No. 60/897,132, filed on Nov. 12, 2007, which is herebyincorporated by reference herein in its entirety.

FIELD

This application relates generally to the conversion of solar radiationto electric power, and, more particularly, to maintaining apredetermined thermal profile of a solar power receiver in a solarenergy-based power generation system.

SUMMARY

Systems, methods, and devices for providing a predetermined thermalprofile, such as a uniform solar heat flux profile, to a primary solarreceiver(s) (or receiver section(s)) in a solar energy-based powergeneration system are disclosed herein.

An example of a solar energy collection system includes a primary solarreceiver and a secondary solar receiver. The secondary solar receivercan generate steam using energy from solar radiation incident thereon.The primary solar receiver can receive the generated steam from thesecondary solar receiver and superheating the steam using energy fromsolar radiation incident thereon. A plurality of heliostat-mountedmirrors can be disposed so as to reflect incident solar radiation at oneof the primary and secondary solar receivers. A controller can beconfigured to control the plurality of heliostat-mounted mirrors. Thecontroller can be further configured to aim at least a portion of theplurality of heliostat-mounted mirrors at the primary solar receiver soas to provide a predetermined thermal profile on a surface of theprimary solar receiver.

The predetermined thermal profile can include a uniform solar heat fluxon the surface of the primary solar receiver, a predeterminedtemperature profile on the surface of the primary solar receiver, or apredetermined heat flux profile on the surface of the primary solarreceiver.

Preferably, the primary solar receiver operates at a higher temperaturethan the secondary solar receiver.

The solar energy collection system can also include a turbine electricpower plant configured to use the superheated steam from the primarysolar receiver.

The primary solar receiver may be arranged on top of a first tower andthe secondary solar receiver may be arranged on top of a second tower.Alternatively, the primary solar receiver and the secondary solarreceiver may be arranged on the same tower. The primary solar receivermay be arranged lower on the same tower than the secondary solarreceiver.

The controller may be configured to receive feedback regarding a stateof a thermal profile on the surface of the primary solar receiver and toadjust the aiming of the plurality of heliostat-mounted mirrors at theprimary solar receiver in response thereto.

An example of a method for controlling a central solar concentratingenergy system may include determining an amount of energy directable byeach of a plurality of heliostats onto each of a primary solar receiverand a secondary solar receiver located at different positions. Theprimary solar receiver and secondary solar receiver can each have heatexchangers configured to heat a working fluid to thereby power agenerator. The method may further include, at a first time, selecting afirst portion of the plurality of heliostats and aiming the selectedfirst portion at the primary solar receiver responsively to a result ofthe determining. At a second time, the determining may be repeated. Themethod may further include, at the second time, selecting a secondportion of the plurality of heliostats different from said first portionand aiming the selected second portion at the primary solar receiver.Each selecting may be based on a desired thermal profile for a surfaceof the primary solar receiver.

The desired thermal profile may include a predetermined temperatureprofile on the surface of the primary solar receiver, a predeterminedheat flux profile on the surface of the primary solar receiver, or auniform solar heat flux on the surface of the primary solar receiver.

Preferably, the primary solar receiver has a higher operatingtemperature than the secondary solar receiver.

The selecting at the second time may include determining a uniformity oftemperature or energy flux on the surface of the primary solar receiver.The aiming at the second time may be responsive to the determining theuniformity.

The repeating the determining at the second time may be responsive to afeedback signal indicating a change in a thermal profile for the surfaceof the primary solar receiver.

In an example of a method for controlling heliostat-mounted mirrors in asolar energy collection system, the solar energy collection system mayinclude a primary receiver, a secondary receiver, and a plurality ofheliostat-mounted reflectors. The primary receiver may have morestringent operating criteria than the secondary receiver. Eachheliostat-mounted reflector can be configured to direct incident solarradiation onto one of the primary and secondary receivers.

The method may include observing a state of the primary receiver,comparing the observed state to a predetermined state for the primaryreceiver, and directing at least a portion of the heliostat-mountedreflectors onto or away from the primary receiver based at least in parton said comparing. The predetermined state can be related to theoperating criteria for the primary receiver.

The predetermined state can include a predetermined temperature profileof a surface of the primary receiver or a uniform heat flux profile on asurface of the primary receiver.

Preferably, the primary receiver operates at a higher temperature thanthe secondary receiver.

Objects, advantages and novel features of the present disclosure willbecome apparent from the following detailed description.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1A is a diagrammatic elevation view of a solar power tower with tworeceivers in a field of heliostats.

FIG. 1B is a diagrammatic elevation view of a solar power tower withmultiple receivers in a field of heliostats.

FIG. 2A is a simplified diagram showing a layout of various solarreceivers with associated heliostats in a turbine based solar powersystem.

FIG. 2B is a diagrammatic elevation view of a pair of solar power towerswith different receivers in a field of heliostats.

FIG. 3A is a simplified diagrammatic plan view of a solar power tower ina field of heliostats.

FIG. 3B is a simplified diagrammatic plan view of two solar power towerswith different receivers in a field of heliostats.

DETAILED DESCRIPTION

In general, the present disclosure is directed to methods, systems, anddevices for providing a uniform and/or accurate solar heat flux profileto a primary solar receiver(s) (or receiver section(s)) in a solarenergy-based power generation system. A solar energy-based powergeneration system can convert solar energy to thermal energy and use thethermal energy to drive a generator. In the illustrative embodimentsdescribed herein, a working fluid, for example, steam, can be employedto power a prime mover, preferably a turbine operating on a Rankinecycle. The prime mover, which may be accompanied by additional primemovers in a single system, may be used for electric power generation,pumping, or any other suitable purpose. Preferably, the prime mover ormovers is/are connected directly to a generator for electric powergeneration. One or more solar receivers are employed to receiveconcentrated sunlight from heliostats and convert the sunlight to heat.In some embodiments, the working fluid, such as water/steam, iscirculated through the receiver. In other embodiments, a heat transferfluid is circulated which transfers heat to the working fluid via anintermediate heat exchanger.

It is generally desirable to maintain a lower level of heat flux on theexternal surface of receivers (or receiver sections) in which a workingfluid is heated to a higher temperature, compared with receivers inwhich a fluid is heated to a lower temperature. Among other reasons,this helps to avoid localized hotspots, where the margin for error inmaterials' design limits may be more easily breached than in lowertemperature regions. For materials typically employed in solar boilerand/or receiver construction, the material strength decreases withincreasing temperature. As receiver material temperatures increase,internal stresses generated by the working fluid (or other factors) onthe receiver material could exceed the tensile strength thereof,potentially leading to catastrophic failure. Both the absolutetemperature and the temperature gradient are concerns in this regard.

For example, a working fluid can be heated to a first temperature in therange of 350-500° C. in a secondary receiver (or receiver section). Theworking fluid from the secondary receiver may subsequently be furtherheated to a temperature in the range of 500-700° C. in a primaryreceiver (or receiver section). To achieve these working fluidtemperatures, the secondary receiver (or receiver section) may have adesired solar heat flux on an external surface thereof in the range of200-600 kW/m² while the primary receiver (or receiver section) may havea desired solar heat flux on an external surface thereof in the range of100-200 kW/m². In another example, the working fluid in the primaryreceiver (or receiver section) is at a lower pressure than the fluid inthe secondary receiver (or receiver section), for example, a reheatingreceiver. In such an example, the desired solar heat flux may be in therange of 80-130 kW/m². Note that the primary receiver (or receiversection) may be any portion that operates at a higher temperature or isotherwise more sensitive to temperature gradients (gradients taken overthe flux receiving surface). For example, the primary receiver may serveto superheat the working fluid of a solar thermal plant. Another exampleis a reheater receiver. Both would operate at a lower heat flux (energyper unit area) than a secondary receiver (or receiver section), whichmay serve, for example, to preheat and/or evaporate the working fluid.

In the primary receiver (or receiver portion), there is a need toachieve greater uniformity of solar heat flux. The goal of temperatureuniformity includes maximizing the working fluid temperature bymaximizing the flux on the receiver without exceeding the local absolutetemperature limits and temperature gradient limits of the receiver atany point. These requirements can translate to a single parameterdefining a goal condition of a cost function which can be minimizedthrough the control of the heliostats. In an embodiment, the costfunction is employed in a function-minimization algorithm.

In an embodiment, each receiver portion (primary and secondary—orfurther) has a fixed number of target, or aiming, points on its surface.The function minimization algorithm assigns each heliostat to arespective target point based on the time of day, insolation levels,cloud cover, season of the year and any other variables that can bedetected to characterize a current operating environment. For thepresent discussion, since it is the most sensitive to flux, we refer tothe primary receiver portion, only. The primary receiver is givenpriority in terms of the heliostat allocation in order to define theheliostat-to-target point assignments that correspond to the minimumcost. In a first method, the cost function may be, for example, the sumof the difference between a peak allowed temperature for the receiverportion at a point on the receiver surface (which may vary over thesurface of the receiver) and the measured or predicted temperature atthat point, taken over the entire surface. A measured temperature may beused, but since conditions may vary quickly in a real system, apredicted temperature may be employed for the algorithm instead, thelatter being derived from an accurate model of the system capable ofproviding a steady state or unsteady state temperature prediction basedon current operating conditions (such schemes that rely on predictiveconditions rather than measured are often referred to as “model-basedcontrol”). The negative effect of high gradients may be included in thefunction by including a variable multiplier for each temperaturedifference that increases with the magnitude of the temperaturedifference in the sum, for example by squaring the temperaturedifference.

Σ(T_(i,P)−T_(i,M))²,

where T_(i,P) is the predicted (or measured) temperature of the ipositions of the receiver surface and T_(i,M) is the maximum temperatureof the i position of the receiver surface. Alternatively, astep-function F may provide a multiplier to the temperature differenceand have a high where the predicted or measured temperature is more thana threshold magnitude below the maximum temperature.

ΣF(T_(i,P)−T_(i,M))·(T_(i,P)−T_(i,M))

The function F may also depend on the absolute temperature since thetolerance of the receiver to temperature gradients may be a function ofthe absolute temperature as well. To make the temperature prediction,the spot size and intensity pattern has to be predicted for the localsurface corresponding to the aiming point for the particular heliostat.To do this, preferably, the performance characteristics of eachheliostat are obtained, for example from measuring the lightdistribution on a reference surface using a reference light source, andthis data translated into a set of characterizing data. For example, theprecise shape of the surface can be stored as a triangle mesh ornonuniform rational B-spline (NURBS) model that can be used withray-tracing to predict the beam image on the target based on currentconditions. Preferably, current conditions are predicted using a seasonand time of day model and augmented based on insolation measurements toaccount for cloud cover and other atmospheric conditions.

Another type of cost function may account for the penalty ofnonuniformity on the primary receiver by employing a step-functionmultiplier that defines limits on the non-uniformity but within thoselimits, calculates a negative revenue for the instant operatingcondition. By minimizing the negative instantaneous revenue, the costfunction may not seek to achieve the highest possible temperature of theprimary receiver, but may still satisfy, according to the boundarycondition, a uniformity criterion.

To increase the speed and lower the calculation burden, variousheuristics may be used to reduce the size of the assignment optimizationspace. For example, certain heliostats would never be assigned tocertain aiming points due to strongly negative cosine losses,occultation losses, or because of the distance from the heliostat to theaiming point. So some assignments may simply be removed from the list ofassignments to model. Other heuristics may be employed to speed thealgorithm such as baseline assignment maps for a set of standardconditions (e.g., based on time, season, and average insolation level).These assignments would be assumed and then additional assignments madeto bring the flux up to the optimum level using unassigned heliostats.Another way to reduce the computational burden is to assign heliostatsin predefined assignment groups to create a rough baseline and thenassign further heliostats using the cost function minimizationalgorithm.

Each heliostat may be made to have a more uniform flux on a primaryreceiver (or receiver section) by prioritizing the assignment ofheliostats or groups of heliostats to aiming points of the primaryreceiver (or receiver sections) over the assignment of heliostats toaiming points on the secondary receivers (or receiver sections). Thisprioritization is optimally included in a set of operating procedures ofthe system and/or programmed into a computerized control system. Byprioritizing the assignment of heliostats to the primary receiver, theuniformity may be optimized or uniformity may be held within apredefined range because the pool of available heliostat assignments isgreatest during this phase of the assigning process and therefore thecriteria for assignment of heliostats to the primary receiver do nothave to be as compromised in the resulting schedule of assignments. Thismay result in an optimization of uniformity or merely ensure that someuniformity threshold criterion is met, according to the scheme selected.

With reference to FIG. 1A, an example of a solar energy-based powergeneration system 100 can include a solar receiver 102, which can be atarget for solar radiation reflected thereonto by heliostats 110 for thepurpose of heating a working fluid. The receiver 102 can be located atthe top of a single solar power tower 108, or at some other location,for example, if an intermediate reflector is used to bounce lightreceived at the top of a tower down to a receiver located at groundlevel. The receiver 102 may include members for containing a fluid, suchas tubes, conduits or cavities, and may also include elements forconveying a fluid to and from these members, such as pipes, ducts,channels or headers.

Each heliostat in the field 110 can track the sun so as to reflect lightonto the receiver 102 in the tower 108. Heliostats can be arrayed in anysuitable manner, but preferably their spacing and positioning areselected to provide optimal financial return over a life cycle accordingto predictive weather data and at least one optimization goal such astotal solar energy utilization, energy storage, electricity production,or revenue generation from sales of electricity.

As shown in FIG. 1A, the single-tower system 100 can include multiplereceivers, for example, a primary receiver 106 and a secondary receiver104, on a single tower 108, where each receiver can have a differentfunctionality. In general, the primary receiver 106 requires a moreuniform and/or more accurate heat flux profile on its surface than thesecondary receiver 104 due to a higher operating temperature and/orlower flux requirement. For example, the primary receiver 106 may beused to superheat steam generated in the secondary receiver 104.Although shown adjacent in FIG. 1A, the primary and secondary receiversmay be spaced apart on the central tower 108. Each heliostat in thefield of heliostats 110 can be directed to focus incident solarradiation onto one of the receivers, as directed by a control system(not shown) or operator. For example, the heliostats closest to thecentral tower 108, such as heliostat 112 can focus radiation 114 ontoprimary receiver 106 while heliostats farther from the central tower108, such as heliostat 116, can focus radiation onto secondary receiver104.

In an alternate configuration shown in FIG. 1B, a single-tower systemcan include three or more receivers, where a secondary receiver 104heats a fluid to a first temperature and a primary receiver 106 heats afluid, including the same fluid, to a second temperature which is higherthan the first. The third receiver 120 can be used for reheating a fluidin a turbine reheat cycle to substantially the second temperature.

Referring now to FIG. 2A, a solar energy based power system can includea preheating stage, an evaporation stage, a superheating stage, and areheating stage. A receiver, a section of a receiver, or multiplereceivers or sections are represented by the box indicated at 25. Forsimplicity this will be referred to as receiver 25. Receiver 25 isconfigured such that a selected quantity of light 31 from a heliostatarray 24A is incident thereon. A controller 30 controls each of theheliostats (not shown individually in this figure) in the arrays24A-24D. Note that heliostat array 24A-24D may all be part of the samefield or separate fields. The array 24A includes a selected set ofheliostats and does not necessarily represent a contiguous array. Thearray 24A is preferably a subset of a larger set of heliostats whichincludes those of arrays 24B, 24C, and 24D. The light 31 heats thereceiver 25 which in turn preheats a working fluid, for example, water,either directly or via a combination of a circulating heat transferfluid and a heat exchanger, which are not shown. The preheated workingfluid is then evaporated in an evaporating stage by the heat collectedby a receiver 26 (which also may be one or more receivers or receiversections). The light 32 from heliostat array 24B is incident on theevaporator receiver 26 and provides the energy for evaporation.Similarly, light 33 from heliostat array 24C is incident on thesuperheating receiver 27 and provides the energy for superheating theevaporated working fluid, for example, steam. The superheated steamdrives a turbine 28. A reheat section 29 can receive radiation 34 fromits own associated heliostat array 24D as well.

Preferably, at least some of the receivers 25-27 and 29 are configuredsuch that they can be selectively illuminated by selected heliostats ofrespective arrays in the manner discussed above. That is, selectedheliostats focused one receiver can be diverted to a primary receiver,such as the superheating receiver 27 or the reheating receiver 29, toachieve any of several objectives. This diversion of heliostats, forexample, from one receiver 26 to another receiver 27, is illustrated inFIG. 2A by the flux indicated figuratively at 36. Note that theembodiment of FIG. 2A may be modified to include more or fewer stagesand the stages may be different, such as for example with a Braytoncycle-based system using air. In the latter case, the stages may betemperature stages.

According to various embodiments, when the ability of certain heliostatsto supply reflected light to a particular receiver is compromised, suchas by transient cloud coverage or heliostat faults, heliostats can bereallocated to achieve specified control goals, such as temperatureuniformity of a high temperature receiver. Any of the arrays 24A through24D may be configured to be divertible in such a manner to achievevarious objectives.

With reference to FIG. 2B, an example of a solar energy-based powergeneration system 200 include multiple towers 204, 208 in a field 210 ofmirror-bearing heliostats. In such a multi-tower system, each heliostatcan change its targeting between the various towers in the system, forexample between towers 204 and 208, depending on operating requirementsand other conditions. A primary receiver 202 can be located at the topof a first tower 204 while a secondary receiver 206 can be located atthe top of another tower 208. As discussed above with reference to FIG.1A, the primary receiver 202 can require a more uniform and/or moreaccurate heat flux profile on its surface compared to the secondaryreceiver 206. For example, the primary receiver 202 may be used tosuperheat steam generated in the secondary receiver 206.

Each heliostat in the field 210 can track the sun so as to reflect lightonto the receivers 202, 206 in respective towers 204, 208, as directedby a control system (not shown) or operator. For example, the heliostatsclosest to the tower 204 with the primary receiver 202, such asheliostat 212 can focus radiation 214 onto primary receiver 202 whileheliostats further from tower 204, such as heliostat 216, can focusradiation onto secondary receiver 206 in tower 208.

In an alternate configuration, a multi-tower system can include three ormore receivers in respective towers, where a secondary receiver heats afluid to a first temperature and a primary receiver heats a fluid,including the same fluid, to a second temperature which is higher thanthe first. The third receiver can be used for reheating a fluid in aturbine reheat cycle to substantially the second temperature. In anotherexample, similar to the single tower embodiment discussed above, amulti-tower system can also include at least one tower with multiplereceivers. In yet another example, the at least one tower with multiplereceivers can use at least one receiver for reheating a fluid in aturbine reheat cycle.

Because of the large number of heliostats, which could be on the orderof tens of thousands, deployed in a commercial solar power tower system,the heliostats are optimally controlled by a computerized control systemand directed thereby to aim, or focus, reflected solar radiation onspecific aiming points on specific receivers. The aiming points sodirected change depending on operating requirements and otherconditions.

In a multi-tower system, each heliostat can be assigned to tracksunlight onto a different tower, or to the tower to which it is normallyassigned, or to any other tower. The normal assignment may be providedin a normal assignment table in one or more digital controls. Theschedule may contain predefined assignments of heliostats to receiversaccording to time of day and/or time of year. Thus, on a typicalcloudless day in which all heliostats are operative, for a given time ofday and a given time of year, each heliostat can be assigned to trackthe sun such that the light it captures is reflected onto a particularassigned receiver. The location of the heliostat relative to thereceiver may not necessarily indicate the receiver to which it isassigned since interleaving rows of heliostats in adjacent fieldsproduces certain real estate utilization advantages.

Also, preferably, each switchable heliostat is equipped to receiveelectronically or mechanically transmitted instructions to switchreceivers, the instructions being transmitted over wires or fiber opticcables, or wirelessly, or, alternately, through physical action oradjustment made by an operator who is physically present from time totime at each heliostat.

Also, preferably, a control system is provided for transmittingdirection-fixing or direction-changing instructions to at least oneheliostat, causing the at least one heliostat to switch reflective focusfrom one solar receiver to another, including from the tower to whichthe heliostats are normally assigned to a second tower, or back to thetower to which the heliostats are normally assigned from a second tower,or from one tower to another when neither of them is the tower to whichthe heliostats are normally assigned. In yet another particularlypreferred embodiment, the control system transmits simultaneousdirection-fixing or direction-changing instructions to a plurality ofheliostats in one or more of the heliostat fields. In accordance with afurther particularly preferred embodiment, the control system transmitssimultaneous direction-fixing or direction-changing instructions to allheliostats in all of the heliostat fields.

The transmitted instructions may include, but not exhaustively,information regarding when a switch is to be made, for how long eachheliostat is to remain switched, and to where each switched heliostatwill be switched next. Heliostat or heliostat-mounted control systemsmay be equipped to store such instructions for later execution and/ordata retrieval.

The control system may provide instructions based on previouslyprogrammed field configurations, on ad-hoc or single-use configurations,or on up-to-the-minute calculations of field and system parameters thatmay include, but not exhaustively, instantaneous and cumulative solarflux, other climatic conditions and measurements, receiver inlet andoutlet temperatures and pressures, receiver heat flux measurements, timeof day, day of the year, differential electricity tariff's, regulatoryfossil fuel allowances and restrictions, power purchase agreements withelectric companies, revenue targets, and maintenance requirements, wherethe calculations are dynamically performed by an operator with orwithout the aid of a computerized performance model or alternatively bya computerized performance model without the intervention of anoperator.

In a multi-tower system, the functionality of one receiver may bedifferent than that of another. For example, one or more secondaryreceivers may be used to heat a fluid to a first temperature, and aprimary receiver used to further heat the same fluid (or a differentfluid) to a second temperature which is higher than the first.Similarly, in a single-tower system, a receiver may be divided intosections with different functionalities. For example, in the northernhemisphere, the northern section (i.e., secondary receiver section) of acentral receiver may be used to heat a fluid to a first temperature, andthe southern section (i.e., primary receiver section) of the samecentral receiver used to further heat a fluid to a second temperaturehigher than the first. This example criterion may be modified based ongeographic location. For example, the above-criterion may be altered fora geographic location in the southern hemisphere versus a location inthe northern hemisphere. The fluid-containing members andfluid-conveying elements are designed and positioned consistent with thepressures, temperatures, flow rates and fluid composition desired in therespective receivers or receiver sections.

According to an embodiment, a method of operating a power tower systemincludes assigning heliostats to target points on the primary receiverto optimize flux distribution on the primary receiver (or section of areceiver) and only then on at one or more other receivers, e.g., thesecondary receiver. The assigning is optimally performed by acomputerized control system and/or a system operator. The heliostatsselected for the primary receiver may form a contiguous ornon-contiguous array, and in another alternative embodiment can be acombination of heliostats forming continuous or non-contiguous groups.In a preferred embodiment, a solar power tower system includes multiplesets of heliostats, and the heliostats assigned to the primary receivercan be from one of the heliostat sets or from more than one. The systemoperator or control system assigns heliostats according with prioritygiven to the criteria that account for heat flux uniformity, and/oradditional criteria as discussed above. According to the method, thespecific heliostats selected to focus on the primary receiver orreceiver section will change during the course of a day and from seasonto season in accordance with the change in the apparent position of thesun in the sky.

In a preferred embodiment, the primary receiver (or receiver section) isthe highest-temperature or lowest-flux receiver or receiver section inthe system. In an especially preferred embodiment, a fluid is heated toa higher temperature in the primary receiver (or receiver section) thanin at least one other receiver (or receiver section), e.g., a secondaryreceiver. In another especially preferred embodiment, a fluid is heatedto a higher temperature in the primary receiver (or receiver section)after it has been heated in at least one other receiver (or receiversection), e.g., a secondary receiver.

In another preferred embodiment, the assignment of heliostats to areceiver and aiming points thereon is based on factors which includepredictive weather data, including insolation data and seasonal andhourly apparent sun position. In an especially preferred embodiment, thepredictive weather data includes at least one of historical weather dataand a weather forecast. In another especially preferred embodiment, thefactors include differential electricity tariffs which place a highervalue on electricity produced at certain times than on electricityproduced at other times. In yet another especially preferred embodiment,the assignment of heliostats is updated periodically or continuously inresponse to measured and/or forecasted environmental conditions, as wellas feedback from the receiver which includes, but not exhaustively,temperature measurements at the receiver, thermal photogrammetry, andlight-intensity photogrammetry.

In an embodiment, the heliostats of the fields are controlled to directlight onto the receivers so as to provide a goal temperature profile (orgoal flux profile) on the primary receiver (or receiver section). Forexample, controlling the heliostats can be focused on providing theprimary receiver with a uniform profile (i.e., variation across thesurface of the receiver within a predetermined level) and/or adherenceto a predetermined profile. In the primary receiver (or receiversection), because of high pressures and temperatures, the highestoperating temperatures may be achieved within channels of materials oflimited mechanical tolerance when there is a specified degree oftemperature uniformity. The degree of uniformity of temperature/flux orthe desired temperature or flux profile will vary based on the design,but in order ensure against hot spots where failures may occur ornon-uniform temperatures which may create thermal stresses due todifferential expansion, a goal temperature profile or uniformity levelmay be defined. By controlling the heliostats such that at leastselected heliostats can be aimed at primary receiver (or receiversection), the ability of a system to achieve the goal state with highutilization efficiency of the heliostats and real estate can beachieved.

The fluid heated in the primary receiver (or receiver section) can be agas or a supercritical fluid. For example, the supercritical fluid canbe steam generated in at least one other receiver (or receiver section),e.g., a secondary receiver, at a pressure of more than 220 bar andconveyed to the primary receiver (or receiver section) where it isfurther heated to a temperature above 600° C. In another example, thesupercritical steam can be further heated in the primary receiver (orreceiver section) to a temperature above 650° C. In a further example,the supercritical steam can be further heated in the primary receiver(or receiver section) to a temperature above 700° C. In yet anotherexample, the supercritical steam can be generated at a pressure of morethan 250 bar.

In another embodiment, the assignment of heliostats for the primaryreceiver (or receiver section) can be made without regard to thepotential impact on flux distribution on the at least one secondaryreceiver (or receiver section). For example, solar radiation may betemporarily reduced in parts of a heliostat field by partial cloudcover. In such a scenario, heliostats can be assigned to achieve optimumflux distribution on a primary receiver (or receiver section) withoutconsidering the potential impact on other factors such as fluxdistribution on the at least one secondary receiver (or receiversection). The reassignment of heliostats to the primary receiver becauseof the temporary partial cloud cover leaves at least one secondaryreceiver with insufficient or poorly distributed reflected solarradiation for properly performing its designed heating function at agiven fluid flow rate. The fluid flow rate in that at least onesecondary receiver may thereupon be reduced until such time as therequired level of insolation, or substantially uniform distributionthereof, is restored. In another example, the insufficient or poorlydistributed reflected solar radiation at the at least one secondaryreceiver may cause a localized hotspot on the surface of that secondaryreceiver.

In a preferred embodiment, the optimization of flux distribution on theprimary receiver (or receiver section) can be performed either on thebasis of uniformity of flux distribution, including achieving minimumflux differences, or on the basis of accurate adherence to a preselectedset of values, including maximum flux values. Flux may be measured orcalculated by methods that include, but not exhaustively, temperaturemeasurements at or internal to the receiver, thermal photogrammetry, andlight-intensity photogrammetry.

In another preferred embodiment, the optimization can be performedeither on the basis of uniformity of temperature distribution, includingachieving minimum temperature differences, or on the basis of accurateadherence to a preselected set of values, including maximum temperaturevalues.

According to another embodiment, a method for operating a power towersystem includes assigning heliostats to optimize flux distribution firston two primary receivers (or receiver sections), and only then on atleast one secondary receiver (or receiver section). In a preferredembodiment, the two primary receivers (or receiver sections) are thehighest-temperature and/or lowest-flux receivers (or receiver sections)in the system. In an especially preferred embodiment, a first of the twoprimary receivers (or receiver sections) is used for heating to a highertemperature a working fluid previously heated to a first temperature inthe at least one secondary receiver (or receiver section). A second ofthe two primary receivers can be used for reheating a fluid extractedfrom a turbine with a reheat cycle. In another especially preferredembodiment, the second of the two primary receivers (or receiversection) used for reheating can be the lowest-flux receiver (or receiversection) of any receiver in the solar-energy power system. Furthermore,the second of the two primary receivers (or receiver section) used forreheating can be optimized before the first of the two primary receivers(or receiver sections), i.e., before the one used for heating to ahigher temperature a working fluid previously heated to a firsttemperature in the at least one secondary receiver (or receiversection).

In another embodiment, a control system provided for operation in asolar-energy power system can be configured to assign heliostats tooptimize flux distribution first on a primary receiver (or receiversection) and only then on at least one secondary receiver (or receiversection). In alternative embodiments, the control system can beprogrammable and can be programmed with flux or temperature optimizationgoals that give priority to at least one primary receiver (or receiversection) over at least one other receiver (or receiver section).

In a preferred embodiment, the control system can include a centralizedcontrol system configured to transmit instructions. The control systemcan also include a plurality of individual heliostat controllers capableof receiving such instructions from the centralized control system. Theinstructions can include focusing on aiming points on specific receivers(or receiver sections). At least some of the instructions can includefocusing on aiming points on a specific receiver (or receiver section)in order to meet/achieve the flux or temperature optimization goalsprogrammed into the control system.

In an embodiment, the control system can employ one or many optimizationalgorithms and/or strategies for determining aiming points of heliostatsto provide a uniform and/or accurate heat flux profile on the primaryreceiver (or receiver section). The optimization algorithm/strategiesmay be performed on all of or only a portion of the heliostats in thefield. The optimization algorithms/strategies may also includepre-ranking of the heliostats based on predetermined criteria for agiven system condition. For example, some pre-ranking may occur based onthe appropriateness of each heliostat to provide a particular heat fluxgiven the time of day and time of year and/or on the quality(uniformity, accuracy, spot size, cosine losses, occultation, etc.) ofthe radiation reflected by each heliostat for the primary receiver. Suchoptimization algorithms and/or strategies can periodically repeat atfixed intervals, for example, every 10 minutes. In an example, theoptimization algorithms and/or strategies are configured to repeatwhenever a non-uniform heat flux profile of the primary receiver isdetected by a feedback mechanism. In another example, the optimizationalgorithms and/or strategies are configured to repeat whenever adeviation from a predetermined profile for the temperature and/or heatflux on the surface of the primary receiver is detected by a feedbackmechanism.

For example, a heuristic algorithm can be employed on a portion of theheliostats in the field to determine the aiming points generating themost uniform and/or most accurate heat flux profile in a givenprocessing time. The heuristic algorithm may be followed by an algorithmoptimizing the uniformity and/or accuracy obtained by the heuristicalgorithm. In effect, the optimizing algorithm “fills in” deficientareas with reflected radiation from the heliostats in the field outsideof the heuristic optimized portion. Alternatively, the heuristicalgorithm may be applied to all of the heliostats in the field. Inanother example, a greedy algorithm can be applied on any or all of theheliostats in the field. In still another example, a functionalminimizing algorithm may be employed with a goal of minimizing a measureof uniformity and/or accuracy by optimizing a heliostat-target vector.In yet another example, a Monte Carlo simulation method may be employedto determine the optimum aiming configuration for the field ofheliostats to obtain a uniform/accurate heat flux profile on the primaryreceiver without consideration of the effects on the secondary receiver.

In another preferred embodiment, the control system can receivesubstantially real-time feedback from at least one feedback mechanism.The at least on feedback mechanism can include, but is not limited to,temperature measurements at or within the receiver(s), thermalphotogrammetry, and light-intensity photogrammetry. In one example,temperature and/or flux of the primary receiver (or receiver section)can be measured in real time and used to control heliostats to adjustthe heat flux on the primary receiver (or receiver section) to a desiredprofile. For example, infrared and/or optical video can be used to seeradiation emanating from the primary receiver (or receiver section) soas to judge temperature and/or flux parameters. Also, temperaturesensors, such as thermocouples, may be embedded in the primary receiver(or receiver section) and used as control inputs for controlling theheliostats. In order to control the heliostats to correct a measuredconfiguration or seek a goal configuration, a model-based control systemcan be employed that obtains inputs, such as the level of insolation oneach respective heliostat, the pattern of light that is reflected byeach heliostat for a given angle of incidence and aiming, and otherparameters, which may be stored in a memory.

In yet another preferred embodiment, the control system can beprogrammed to use predictive weather data, including insolation data andseasonal and hourly apparent sun position, and at least one optimizationgoal such as total solar energy utilization, energy storage, electricityproduction, or revenue generation from sales of electricity, forformulating the instructions that are transmitted to heliostats. In anespecially preferred embodiment, the predictive weather data includes atleast one of historical weather data and a weather forecast.

As discussed above, a principal criterion for optimization is that solarradiation reflected onto the external surface of the primary receiverconforms to some goal flux distribution or temperature distributionprofile. This uniform distribution allows for precise control of theconditions of the primary receiver. For example, precise control of theprimary receiver may enable precise control of superheated steamconditions, which may be necessary for efficient use of the steam by asteam turbine electric power plant. Variations in the solar heat fluxincident on the surface of the primary receiver (i.e., a non-uniformheat flux) may affect the state of the working fluid exiting therefrom,which, in turn, could adversely affect power generation efficiency.Further, variations in the solar heat flux may lead to localizedtemperature variations (i.e. hot spots) on the surface of the primaryreceiver. These hot spots may have a temperature exceeding the failurethreshold for the primary receiver material. In addition, thetemperature variation between adjacent portions of the primary receivermay induce thermal stresses due to differing amounts of thermalexpansion that could lead to eventual failure. By ensuring a uniformheat flux distribution, the highest possible temperature can be achievedfor the working fluid (e.g., superheated steam) without localized hotspots that may damage the primary receiver. In accordance with thisgoal, an exemplary embodiment of the present invention seeks to maintaina uniform heat flux distribution on the primary receiver by dynamicallycontrolling the focal points of heliostats.

FIG. 3A shows an example of a solar energy-based power system having asingle tower 302 including at least a primary receiver (or receiversection) and a secondary receiver (or receiver section). Surrounding thetower 302 is a field 304 of heliostats (individual heliostats notshown). Although the field 304 is shown to be square in FIG. 3, anyshape may be employed for the heliostat field, such as, but not limitedto, oval, polygonal, crescent-shaped, and other non-regular geometricshapes. Furthermore, although the tower 302 is shown centered in thefield 304, the tower 302 may be located elsewhere in the field, such asoff-center, or outside of field 304.

As discussed above, the primary receiver in tower 302 requires moreuniform and/or accurate heat flux profile on a surface thereof due to ahigher operating temperature or a lower heat flux requirement comparedto the secondary receiver. The focal direction for each heliostat in thefield 304 is dynamically configurable such that the radiation reflectedfrom a particular heliostat may be directed from the secondary receiver(or receiver section) to the primary receiver (or receiver section) andvice-versa.

Controller 316 controls the focal direction for the heliostats in field304. Although only a single controller is shown, separate controllersmay be provided for portions of the heliostat field 304 and/or for eachheliostat in the field. Controller 316 can monitor the heat flux on theprimary receiver (or receiver section) in tower 302 and gauge heat fluxuniformity and/or adherence to predetermined flux or temperatureprofile. Alternately, a separate system may be provided which monitorsthe heat flux distribution on the primary receiver and conveys suchinformation through feedback 318 to the controller 316 for subsequentoptimization of the focal direction of the heliostats in the field 304.

Monitoring the heat flux and/or temperature distribution on the primaryreceiver may be achieved by a variety of techniques. For example,thermocouples or other temperature sensors may be affixed to the surfaceof the primary receiver or in the vicinity of the surface of the primaryreceiver. Variations in the temperature may then be correlated tovariations in incident heat flux on the primary receiver. In anotherexample, remote temperature or heat flux measurements of the primaryreceiver surface may be obtained using, for example, thermal imagingtechniques so as to determine the profile of reflected optical energy.

In another example, the radiation incident on each heliostat in thefield 304 may be monitored using photometric techniques. An imagecapture system, for example, one employing a digital camera, can captureimages of reflectors that can be mounted on some or all of theheliostats in the field or of reflectors that can be positioned atvarious points in the heliostat field. The reflectors may be diffusereflectors having a selective surface coating or color that makes theirimages easier to separate from any background image or noise. Thequantity of light falling on the reflectors can be determined byappropriate optical and digital filtering and image processing usingsuitable techniques.

Additionally or alternatively, each heliostat or group of heliostats inthe field may be provided with an insolation level detector, such as apyranometer or pyrheliometer. The insolation level detectors may bemounted in the center of the heliostat or at another location thereon soas to provide a measure of the solar energy incident on each heliostat.Alternatively, a plurality of insolation level detectors may bestrategically placed throughout the field of heliostats, whereby theinsolation level detected by each detector is correlated with a numberof the heliostats surrounding it.

In the event of a disruption to the uniform insolation distribution onthe primary receiver (or receiver section), heliostats may be reorientedto compensate for the disruption. The disruption may be due tomechanical failure of one or more heliostats in the field, occultationdue to cloud coverage or other shadow effects, fouling of the reflectivesurface of one or more heliostats, changes in time of day and/or time ofyear, as well as other causes. Note that the disruption may also bepredicted by a predictive control algorithm when the loss of reflectedradiation from heliostats is a pre-determinable event. The controller316 may prioritize the reorientation of the focal points of the entirefield or a portion of the field of heliostats to compensate for thenonuniformity on the primary receiver.

For example, in a simplified example, suppose that a portion of theheliostat field is directed at the primary receiver while the remainderof the heliostat field is directed at the secondary receiver. During adisruption, the heliostats currently directed at the primary receivermay be unable to maintain a uniform temperature profile on the primaryreceiver. This may be determined by measurement of instantaneous energyfalling on the heliostats or inferentially by a change in the measuretemperature and/or flux on the primary receiver. As a result of thevariation of the heat flux profile on the primary receiver, thecontroller determines that additional heliostats are needed to maintainthe uniformity. The controller may calculate a pair of demarcation lines306 extending radially from tower 302. The demarcation lines 306 may bedrawn such that they border encompass a region of the primary receiversurface determined to have a non-uniform heat flux profile.Alternatively, the demarcation lines may be drawn to border a region ofthe heliostat field 304 known to be causing the non-uniformity on theprimary receiver.

The demarcation lines 306 define a candidate region of heliostats in thefield 304. This candidate region of heliostats may be furthersub-divided into a plurality of groups. For example, the candidateregion may be sub-divided into groups 308, 310, 312, and 314, eachprogressively farther from the tower 302. The number of groups has beenchosen merely for illustrative purposes, as any number of groups in thefield of heliostats may be used. It is also noted that the sub-divisioninto groups based on distance is merely for illustrative purposes. Othergrouping schemes may also be employed. For example, the field ofheliostats may be sub-divided based on, but not limited to, focused spotsize on the primary receiver surface, focused spot uniformity on theprimary receiver surface, cosine losses, accuracy of the focused beam,heliostat location with respect to the primary receiver and/ornonuniformity, and predictability.

Controller 316 may select group 308 closest to the tower 302. For theheliostats within the group 308, the controller 316 determines theamount of heat flux from each heliostat that would be directed on theprimary receiver if the heliostats were redirected to focus on thenon-uniform region thereof. Based on this calculation, the controller316 can select any or all heliostats from group 308 for redirection tothe primary receiver so as to balance the non-uniform flux region. Inthe event that the heat flux newly directed on the primary receiver bygroup 308 is insufficient to overcome the measured non-uniformity, thecontroller 316 may repeat the analysis and redirection with group 310.Likewise, if the combined heat flux from groups 308 and 310 isinsufficient, the controller may repeat the process with group 312. Thisprocess may continue with additional groups arranged at distancesprogressively farther from the tower 302 and within demarcation lines306 until the non-uniformity in the heat flux distribution is correctedor until some predetermined criteria is met.

A similar process may be performed for a multi-tower example, as shownin FIG. 3B. Tower 302 a includes a primary receiver while tower 320 hasa secondary receiver. Surrounding both towers is a field 304 a ofheliostats. Controller 316 controls the focal direction for theheliostats in field 304 a. Controller 316 can monitor the heat flux onthe primary receiver (or receiver section) in tower 302 a and gaugesheat flux uniformity and/or adherence to predetermined flux profile. Asdiscussed above with regard to the single tower example, to correct anonuniformity measured in the primary receiver in tower 302 a,controller 316 may redirect heliostats in group 308 a defined bydemarcation lines 306 a. However, controller 316 may also redirectheliostats in group 322 proximal to tower 320, which may previously havebeen directed at the secondary receiver in tower 320, onto the primaryreceiver in tower 302 a.

Note that although according to the above discussion, adjacentheliostats are re-aimed to compensate for loss of adjacent heliostats,the lost heliostats and the redirected heliostats need not lie inadjacent groups. The controller may be provided with sufficientinformation to “cherry-pick” the most suitable heliostats to use forcompensation. Failures or variations in flux on a surface of the primaryreceiver need not be generated by contiguous portions of the heliostatfields.

It is noted that the above description relates to directing heliostatsonto the primary receiver when an insolation or heliostat reflectiondeficiency leads to the detection of a non-uniform heat flux or adeviation from a predetermined flux or temperature profile. However,variations in insolation or other conditions may cause portions of theprimary receiver to exceed a predetermined value (i.e., a hot spot) aswell. In such a case, the controller 316 may redirect heliostats focusedon the primary receiver to other focal points, such as the secondaryreceiver or an idle position. As above, demarcation lines 306 may bedrawn to define a region of heliostats for potential redirection. Theseheliostats may be divided into groups based on distance from the tower302, wherein the controller 316 individually controls the heliostatswithin each group. The controller may select a group farthest from thetower 302 and determine the amount of heat flux from each heliostatwithin the selected group. Based on this calculation, the controller 316can then choose certain heliostats from the selected group forredirection. These heliostats may be redirected to a different tower(e.g., tower 320 in FIG. 3B) or away from any tower (e.g., the sky).

The controller 316 may also command all of the heliostats from theselected group to redirect away from the primary receiver based on thedegree of non-uniformity. In the event that the non-uniformity remainsafter redirection, the controller may then proceed to the next closestgroup and repeat the above sequence. This process may continue withadditional groups within the demarcation lines and progressively closerto the primary receiver until the non-uniformity in the heat fluxdistribution is corrected or until some predetermined condition isreached.

Other control and optimization techniques are contemplated as well.Controller 316 can employ one or many optimization algorithms and/orstrategies in order to determine aiming points on a surface of theprimary receiver (or receiver section) for heliostats so as to provide auniform and/or accurate heat flux profile thereon. Characteristics ofthe heliostats employed in the optimization algorithms and/or strategiesmay be stored in the controller or calculated on the fly. In an example,the shape of a beam spot on the surface of the primary receiver for eachheliostat may be stored in the controller. In another example, theexpected beam projection on the surface of the primary receiver for eachheliostat may be determined from stored information. In yet anotherexample, the controller may calculate the expected beam projection foreach heliostat based on solar position (time of day, time of year) aswell as expected or observed occultation.

In an embodiment, the controller may employ a heuristic algorithm. Forexample, a field of heliostats may be divided into a first group and asecond group. The first group of heliostats may be selected based on thepredicted beam spot characteristics of the reflected radiation of eachheliostat on the surface of the primary receiver. For example, thoseheliostats having the most concentrated beam spots on the surface of theprimary receiver from the available heliostats may be selected for thefirst group. In another example, those heliostats having the mostuniform and/or the most predictable spot characteristics for the primaryreceiver may be selected. In still another example, those heliostatsclosest to the primary receiver may be selected. In yet another example,those heliostats having the lowest cosine losses for the primaryreceiver for the given time of day and year may be selected for thefirst group. Any or all heliostats of the first group may be subjectedto optimization by a heuristic algorithm, which employs stored orcalculated beam characteristics for each heliostat to determine fluxcharacteristics on the primary receiver. After optimization of thetargets of the first group on the primary receiver, an optimizationalgorithm may be employed on the second group of the heliostats to fillin deficient flux areas on the primary receiver. For example, thoseheliostats in the second group having the greatest flux may be directedto regions where the flux on the primary receiver generated by the firstgroup is deficient. This heuristic optimization process may be repeatedat predetermined intervals, for example 10 minutes, or when conditionsdictate that optimization is necessary.

In another embodiment, the controller may employ a meta-heuristicalgorithm, such as a greedy algorithm, a random optimization algorithm,or a genetic algorithm. For example, the controller may employ a greedyalgorithm which makes a locally optimal choice for a heliostat aiming onthe primary receiver. Thus, the controller can assign to a target on theprimary receiver surface a heliostat which may have the most uniformintensity. The controller would iterate this process until all targetson the primary receiver surface are accounted for. Other criteria may beused for assigning a heliostat to a particular target, such as spotsize, predictability, degree of cosine losses, etc.

In another embodiment, the controller may employ a functional minimizingalgorithm. As known in the art, a functional provides a map from avector space to an underlying real number field. For example, anassignment vector representing targets for a field of heliostats or aportion of the field of heliostats can be optimized by applying a bruteforce algorithm with the goal of minimizing a measure of uniformity (oradherence to a predetermined profile) of the heat flux on the primaryreceiver (or receiver section) on the entire assignment vector.

In still another embodiment, the controller may employ a Monte Carlomethod to optimize heliostat aiming for the primary receiver so as toobtain a uniform heat flux profile thereon. As known in the art, MonteCarlo methods are stochastic in that they employ computationalalgorithms that rely on random sampling to achieve an optimized result.The controller can determine a domain of inputs from heliostatsavailable to be focused on a particular portion or portions of theprimary receiver surface (or receiver section surface). Using MonteCarlo methods, inputs can be randomly generated from this domain. Theseinputs can then be deterministically analyzed with regard to the heatflux profile on the primary receiver surface (or receiver sectionsurface), preferably with regard to the uniformity of the heat fluxprofile on the primary receiver surface (or receiver section surface).In a preferred embodiment, this deterministic analysis occurs withoutregard to the heat flux profile of the secondary receivers. The processcan then be iterated until a vector of aiming points (e.g., targets onthe primary receiver surface) is obtained for the domain of heliostatsthat minimizes the non-uniformity of the heat flux profile on theprimary receiver surface.

It should be appreciated that steps of the present disclosure may berepeated in whole or in part in order to perform the contemplatedoptimization of heliostat aiming on a primary receiver. Further, itshould be appreciated that the steps mentioned above may be performed ona single or distributed processor. Also, the processes, modules, andunits described in the various figures of the embodiments above may bedistributed across multiple computers or systems or may be co-located ina single processor or system.

Embodiments of the method, system, and computer program product fordetermining optimized heliostat aiming on the primary receiver may beimplemented on a general-purpose computer, a special-purpose computer, aprogrammed microprocessor or microcontroller and peripheral integratedcircuit element, an ASIC or other integrated circuit, a digital signalprocessor, a hardwired electronic or logic circuit such as a discreteelement circuit, a programmed logic circuit such as a PLD, PLA, FPGA,PAL, or the like. In general, any process capable of implementing thefunctions or steps described herein can be used to implement embodimentsof the method, system, or computer program product for determiningoptimized heliostat aiming on the primary receiver.

Furthermore, embodiments of the disclosed method, system, and computerprogram product for determining optimized heliostat aiming on theprimary receiver may be readily implemented, fully or partially, insoftware using, for example, object or object-oriented softwaredevelopment environments that provide portable source code that can beused on a variety of computer platforms. Alternatively, embodiments ofthe disclosed method, system, and computer program product fordetermining optimized heliostat aiming on the primary receiver can beimplemented partially or fully in hardware using, for example, standardlogic circuits or a VLSI design. Other hardware or software can be usedto implement embodiments depending on the speed and/or efficiencyrequirements of the systems, the particular function, and/or particularsoftware or hardware system, microprocessor, or microcomputer beingutilized.

Embodiments of the method, system, and computer program product fordetermining optimized heliostat aiming on the primary receiver can beimplemented in hardware and/or software using any known or laterdeveloped systems or structures, devices and/or software by those ofordinary skill in the applicable art from the function descriptionprovided herein and with a general basic knowledge of the computer,solar energy-based power systems, and optimization arts.

Moreover, embodiments of the disclosed method, system, and computerprogram product for determining optimized heliostat aiming on theprimary receiver can be implemented in software executed on a programmedgeneral purpose computer, a special purpose computer, a microprocessor,or the like. Also, the method for determining optimized heliostat aimingon the primary receiver can be implemented as a program embedded on apersonal computer such as a JAVA® or CGI script, as a resource residingon a server or image processing workstation, as a routine embedded in adedicated processing system, or the like. The method and system can alsobe implemented by physically incorporating the method for determiningoptimized heliostat aiming into software and/or hardware systems, suchas the hardware and software systems of solar energy-based power system.

It is, therefore, apparent that there is provided, in accordance withthe present disclosure systems, methods, and devices for providing apredetermined thermal profile to a primary solar receiver(s) or primaryreceiver section(s) in a solar energy-based power generation system.Many alternatives, modifications, and variations are enabled by thepresent disclosure. Features of the disclosed examples can be combined,rearranged, omitted, etc., within the scope of the present disclosure toproduce additional embodiments. Furthermore, certain features of thedisclosed examples may sometimes be used to advantage without acorresponding use of other features. Persons skilled in the art willalso appreciate that the present invention can be practiced by otherthan the described examples, which are presented for purposes ofillustration and not to limit the invention as claimed. Accordingly,Applicants intend to embrace all such alternatives, modifications,equivalents, and variations that are within the spirit and scope of thepresent disclosure.

1. A solar energy collection system comprising: a primary solar receiverand a secondary solar receiver, the secondary solar receiver configuredto generate steam using energy from radiation incident thereon, theprimary solar receiver receiving the generated steam from the secondaryreceiver and superheating the steam using energy from radiation incidentthereon; a plurality of heliostat-mounted mirrors positioned so thateach is able to reflect incident solar radiation at at least one of theprimary and secondary solar receivers; and a controller configured tocontrol the plurality of heliostat-mounted mirrors, wherein thecontroller is configured to aim at least a portion of the plurality ofheliostat-mounted mirrors at the primary solar receiver and to performan optimization of a cost function, which is responsive to a predictedtemperature uniformity of the primary receiver, by assigning each of theheliostats to aiming points on the primary receiver, the optimizationbeing such that at least a predefined thermal profile across the surfaceof the primary receiver is achieved.
 2. The solar energy collectionsystem of claim 1, wherein the predetermined thermal profile includes auniform solar heat flux on the surface of the primary solar receiver. 3.The solar energy collection system of claim 1, wherein the predeterminedthermal profile includes a predetermined temperature profile on thesurface of the primary solar receiver.
 4. The solar energy collectionsystem of claim 1, wherein the predetermined thermal profile includes apredetermined heat flux profile on the surface of the primary solarreceiver.
 5. The solar energy collection system of claim 1, wherein theprimary solar receiver operates at a higher temperature than thesecondary solar receiver.
 6. The solar energy collection system of claim1, further comprising a turbine electric power plant configured to usethe superheated steam from the primary solar receiver.
 7. The solarenergy collection system of claim 1, wherein the primary solar receiveris arranged on top of a first tower and the secondary solar receiverarranged on top of a second tower.
 8. The solar energy collection systemof claim 1, wherein the primary solar receiver and the secondary solarreceiver are arranged on a same tower, the primary solar receiver beinglower on the tower than the secondary solar receiver.
 9. The solarenergy collection system of claim 1, wherein the controller isconfigured to receive feedback regarding a state of a thermal profile onthe surface of the primary solar receiver and to adjust the aiming ofthe plurality of heliostat-mounted mirrors at the primary solar receiverin response thereto.
 10. A method for controlling a central solarconcentrating energy system, comprising: determining an amount of energydirectable by each of a plurality of heliostats onto each of a primarysolar receiver and a secondary solar receiver located at differentpositions; the primary solar receiver and secondary solar receiver eachincluding heat exchangers configured to heat a working fluid to therebypower a generator; at a first time, selecting a first portion of theplurality of heliostats and aiming the selected first portion at theprimary solar receiver responsively to a result of the determining;repeating the determining at a second time; and at the second time,selecting a second portion of the plurality of heliostats different fromsaid first portion and aiming the selected second portion at the primarysolar receiver, wherein each selecting is based on a desired thermalprofile for a surface of the primary solar receiver.
 11. The method ofclaim 10, wherein the desired thermal profile includes a predeterminedtemperature profile on the surface of the primary solar receiver. 12.The method of claim 10, wherein the desired thermal profile includes apredetermined heat flux profile on the surface of the primary solarreceiver.
 13. The method of claim 12, wherein the desired thermalprofile includes a uniform solar heat flux on the surface of the primarysolar receiver.
 14. The method of claim 10, wherein the primary solarreceiver has a higher operating temperature than the secondary solarreceiver.
 15. The method of claim 10, wherein the primary solar receiverhas a higher operating temperature than the secondary solar receiver,the selecting at the second time includes determining a uniformity oftemperature or energy flux on the surface of the primary solar receiver,and the aiming at the second time is responsive to the determining theuniformity.
 16. The method of claim 10, wherein the repeating thedetermining at the second time is responsive to a feedback signalindicating a change in a thermal profile for the surface of the primarysolar receiver.
 17. A method for controlling heliostat-mounted mirrorsin a solar energy collection system, the solar energy collection systemincluding a primary receiver, a secondary receiver, and a plurality ofheliostat-mounted reflectors, the primary receiver having more stringentoperating criteria than the secondary receiver, each heliostat-mountedreflector configured to direct incident solar radiation onto one of theprimary and secondary receivers, the method comprising: observing astate of the primary receiver; comparing the observed state to apredetermined state for the primary receiver, the predetermined statebeing related to the operating criteria for the primary receiver; anddirecting at least a portion of the heliostat-mounted reflectors onto oraway from the primary receiver based at least in part on said comparing.18. The method of claim 17, wherein the predetermined state includes apredetermined temperature profile of a surface of the primary receiver.19. The method of claim 17, wherein the predetermined state includes auniform heat flux profile on a surface of the primary receiver.
 20. Themethod of claim 17, wherein the primary receiver operates at a highertemperature than the secondary receiver.